Method for compensating signal distortions in composite amplifiers

ABSTRACT

A method for compensating signal distortions in multiple transmitting branches entering a composite amplifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/296,710, having a 371 date of Oct. 10, 2008, which published asUS20090163154 and which is a 35 U.S.C. §371 National Phase Applicationfrom PCT/SE2006/050067, filed Apr. 10, 2006, which published asWO2007/117189. The above identified applications and publications areincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a method and a system for compensatingsignal distortions in multiple transmitting branches of a compositeamplifier.

BACKGROUND

Composite amplifiers are amplifiers that contain several, individuallydriven, constituent amplifiers connected to each other and the outputvia special output networks. (Constituent amplifier means a singletransistor, or a parallel combination of transistors, together withsupporting circuitry.) This gives composite amplifiers better efficiencythan single-transistor amplifiers (or amplifiers with severaltransistors driven collectively). Doherty and Chireix amplifiers arewidely known examples of composite amplifiers. They are described in W.H. Doherty, “A new high efficiency power amplifier for modulated waves,”Proc. IRE, vol. 24, no. 9, pp. 1163-1182, September 1936 and in H.Chireix, “High power outphasing modulation”, Proc. IRE, vol. 23, no. 2,pp. 1370-1392, November 1935. The Doherty amplifier can be generalizedto more than two constituent amplifiers, as described in e.g. F. H.Raab, “Efficiency of Doherty RF Power Amplifier Systems”, IEEETransactions on Broadcasting, vol. BC-33, no. 3, September 1987.

Several new high-order (3 or more constituent amplifiers) compositeamplifiers with better efficiency have been disclosed recently in forexample WO 2004/023647, WO 2004/057755, WO 2005/031966 and U.S. Pat. No.5,012,200.

A Doherty amplifier consists of a main amplifier and an auxiliary (peak)amplifier connected to each other and the output via an output network.A prototypical output network that gives Doherty operation consists of amain amplifier connected to a common load via a quarter-wavelength linehaving characteristic impedance equal to the main amplifier's optimalload resistance. The auxiliary amplifier is connected directly to thecommon load. The common load resistance is equal to the parallelconnection of the optimal loads of the main and auxiliary amplifiers.

Doherty Radio Frequency (RF) Power Amplifiers (PAs) are very efficientfor amplitude-modulated signals, since they have lower average sum of RFoutput currents from the transistors than conventional amplifiers.Reduced RF current translates into reduced DC (supply) current sinceclass B (half-wave rectified sine transistor current waveform) orsimilar operation of the constituent transistors is used.

An important property of the Doherty output network is that it allowsthe auxiliary amplifier to influence the RF voltage at the mainamplifier while affecting its own RF voltage, and the output voltage,much less (ideally zero). This means that the auxiliary amplifier'sinput drive can be off at output levels below a transition point withoutconsequence to the output. The quarter wavelength line transforms theload into higher impedance at the main amplifier. This has twoconsequences: 1) the main amplifier's efficiency increases, 2) the mainamplifier reaches saturation at a level well below its maximum outputpower (i.e. the transition point). At levels above the transition point,the auxiliary amplifier keeps the main amplifier voltage at asubstantially constant level. This means that the nonlinearity due tothe main amplifier's saturation can be kept low.

The main amplifier gives a substantially linear output RF current overthe whole amplitude range, while the auxiliary amplifier gives alinearly rising RF current only above the transition point, i.e. anonlinear output current. These two currents also have a phasedifference of 90 degrees. By providing RF current, the auxiliaryamplifier also contributes to the output power in the upper amplituderange.

A Chireix amplifier has a different output network than a Dohertyamplifier, and is traditionally driven with equal amplitudes for bothamplifiers. The term “outphasing”, which describes the key method inChireix amplifiers, generally means the method of obtaining amplitudemodulation by combining two phase-modulated constant-amplitude signals.The phases of these constant-amplitude signals are chosen so that theresult from their vector-summation yields the desired amplitude.

Compensating reactances in the output network of the Chireix amplifierare used to extend the region of high efficiency to lower output powerlevels. An equivalent network can be built with shortened and lengthenedtransmission lines, whose sum should be 0.5 wavelengths.

High-order composite amplifiers (see for example WO 2004/023647, WO2004/057755, WO 2005/031966) generally use combinations of Doherty-likedrive signals (one or more amplifiers are driven only above someamplitude) and Chireix-like drive signals (two of the constituentamplifiers are driven with equal amplitudes in some amplitude range).

Direct IQ-modulation in a transmitter is the direct modulation of acomplex baseband signal to a real, analog, signal at intermediatefrequency (IF) or final RF. The real and imaginary parts of a complexbaseband signal are commonly called (due to their mapping to the RFsignal) In-phase (I) or Quadrature-phase (Q), hence the nameIQ-modulation. Direct IQ-modulation has several advantages, chief ofwhich are the high utilization of the available bandwidth of theDigital-to-Analog Converters (DACs), and that this bandwidth is splitbetween two DACs. Both advantages lower the cost of the DAC system.

Direct IQ-modulators are analog complex-to-real multipliers, i.e. twofour-quadrant analog multipliers coupled to a summing node. Themultiplicands are two 90-degree offset Local Oscillator (LO) signals atthe target frequency. The IQ-modulation process is prone to errors dueto various imbalances and offsets in the LO signals, DC levels, analogcircuitry and DAC outputs. These errors can vary nonlinearly withamplitude and also be frequency-dependent. For conventional amplifiersthey are observable in the output signal. They are also correctable.This is discussed in the article “Digital Precompensation ofImperfections in Quadrature Modulators”, R. Marchesani, IEEE. Trans. onCommunications, vol. 48, no. 4, April 2000, pp. 552-556.

Composite amplifiers are however preferred in many products forefficiency reasons as described above. The composite amplifiers consistsof two or more coupled, individually driven, amplifiers. With oneIQ-modulator for each amplifier the different errors from theIQ-modulators are mixed and can not easily be individually observed inthe transmitter output. Therefore, with a single observation receiverthere will be a residual error due to not being able to individuallyobserve the individual IQ-modulators' errors.

A straightforward solution would be to instead observe the individualconstituent amplifier inputs. This, however, means that two (in the caseof Doherty or Chireix) or more observation receivers or a receiver withseveral switchable inputs must be used, which increases cost. In manysituations, the transmitter output too must be observed anyway, forpurposes of linearization, which then increases the number ofobservation receivers to at least three.

SUMMARY

An object of the invention is to provide an easy and effective methodand system for compensation of signal distortions in multipletransmitting branches entering a composite amplifier.

This is achieved in a method according to claim 1, in a system accordingto claim 9 and in a compensation adjustment means according to claim 17.

Hereby the signal distortions in each transmitting branch entering acomposite amplifier can be derived by observing only the output signalfrom the composite amplifier. No extra observation receivers need to beprovided and hereby an easy method using a less complex observationreceiver is achieved.

Preferably a transmitting branch model comprising information about theparameters that affect signal distortions for each transmitting branchis used to derive the contribution from different parameters in thetransmitting branches to the signal distortion that causes the observederror; and compensating parameters or adjustments to already existingcompensating parameters are provided to each transmitting branchaccordingly in order to decrease the signal distortions. Hereby eachparameter in the transmitting branch that contributes to the signaldistortion can be individually compensated for.

Advantageously derivatives are derived of the output signal in respectof the different compensating parameters by utilising said compositeamplifier model and said transmitting branch models and said derivativesare utilised for adapting said compensating parameters such that theerror in the output signal is minimised. Hereby a flexible way ofcompensating signal distortions in many different types of compositeamplifiers is achieved.

Suitably at least two different input signals levels or frequencies areprovided such that the different transmission branches contribute indifferent amounts to the output signal for the different input signals.Hereby individual constituent amplifiers of the composite amplifier canbe singled out or different sets of constituent amplifiers can beactivated in different amounts in order to simplify the computation ofthe contribution from each transmitting branch to the total error. Byproviding different input signals it is also possible to iterate themethod steps in order to obtain gradually improved compensations of thesignal distortions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows schematically a system according to one embodiment of theinvention.

FIG. 1 b shows schematically the components of the compensationadjustment means.

FIG. 2 is a flow chart of a method according to one embodiment of theinvention.

FIG. 3 shows schematically a system according to another embodiment ofthe invention where the composite amplifier is a Doherty amplifier.

FIG. 4 shows schematically a standard implementation of IQ compensation.

FIG. 5 is a flow chart of a method according to one embodiment of theinvention.

FIG. 6 shows schematically a system according to one embodiment of theinvention where the composite amplifier is a Chireix-Doherty amplifiercomprising three constituent amplifiers.

DETAILED DESCRIPTION OF EMBODIMENTS

In FIG. 1 a a system for compensation of signal distortions according toone embodiment of the invention is schematically illustrated. Thissystem can for example be positioned in a base station for mobilesystems.

The system comprises a composite amplifier 1 into which in thisembodiment a first and a second transmitting branch 3, 5 are input.Signals y1 and y2 are the signal inputs to the composite amplifier 1from the first and second transmitting branches 3, 5 respectively. Inthis example a composite amplifier having two inputs is described butcomposite amplifiers having three, four or even more inputs could alsobe used in this invention. The composite amplifier 1 could be any kindof composite amplifier and it can have different numbers of constituentamplifiers. Doherty and Chireix amplifiers are two commonly knownexamples of composite amplifiers with two constituent amplifiers and twoinputs.

The system comprises a generating means 6 for generating two inputsignals a1, a2 to the transmitting branches 3, 5. An input signal x tothe generating means 6 is further also fed to a compensation adjustmentmeans 17 further described below. The first transmitting branch 3comprises a first TX box 7 directly connected to the first input of thecomposite amplifier 1. The first TX box 7 comprises for example aDigital to analog converter, DAC, and an IQ-modulator. It could alsocomprise amplifiers and filters. The DAC and the IQ-modulator providessome signal distortion to the signal passing the first TX box 7. Belowwe will denote the different factors providing signal distortions asparameters and the parameters could thus for example be IQ errors, timedelay, frequency errors or non linearities. Corresponding second TX box9 is positioned in the second transmitting branch 5 and connected to thesecond input of the composite amplifier 1.

According to the invention the first transmitting branch 3 comprisesfurther a first compensation means 11 connected to the first TX box 7and the second transmitting branch 5 comprises a second compensationmeans 13 connected to the second TX box 9. These compensation means 11,13 provide according to the invention a distortion to the passing signalthat substantially compensates for the signal distortions caused laterin the first TX box 7 and second TX box 9 respectively.

The system comprises also a transmitter observation receiver, TOR, 15connected to an output from the composite amplifier 1. The output signalfrom the composite amplifier is called z. An output 16 from the TOR thatcan be connected to for example an antenna is also indicated in theFigure. According to the invention the system comprises further acompensation adjustment means 17 connected to the TOR 15, the twocompensation means 11, 13 and as said above to the input of thegenerating means 6.

The signal z detected in the TOR 15 will have an error in relation tothe input signal x depending on the signal distortions provided in thefirst and second TX boxes 7, 9 assuming the signal distortions providedin the compensation means 11, 13 initially is zero. The compositeamplifier 1 will also contribute to a signal error but this is normallycompensated for by a digital predistorter provided in the system. Thisis not part of the invention and will not be shown here. The first andsecond TX boxes 7, 9 provide distortions to the signal such that anerror in the output signal z can be derived in the compensationadjustment means 17 when compared to the input signal x, also called anideal output signal. The problem to be solved by this invention is toknow how much the different input signals to the composite amplifier y1and y2 contribute to the total signal error. To be able to compensatefor the signal distortions provided in the TX boxes it is necessary toknow how much each transmitting branch 3, 5 contributes to the outputsignal.

According to the invention a composite amplifier model is provided inthe compensation adjustment means 17. Said model comprises informationof how the composite amplifier 1 is constructed regarding how thedifferent input signals y1 and y2 to the composite amplifier 1contributes to the output signal z and under which input signalconditions. I.e. one of the branches could contribute more under certaininput signal conditions. The input signals x could for example differ inpower levels or frequency. The compensation adjustment means 17 uses thederived error in the output signal z and the composite amplifier modelto derive the contribution from each transmitting branch 3,5 to thesignal error. Hereby the compensation adjustment means 17 knows whichtransmitting branches 3, 5 that need to be compensated and approximatelyhow much. The compensation can then be performed iteratively as will bediscussed below.

The signal distortions are substantially compensated in the compensatingmeans 11, 13 according to the already derived signal error contributionsfrom the different branches 3,5. To do this a transmitting branch modelis used for each transmitting branch 3, 5. This transmitting branchmodel comprises information about the contribution to the signaldistortions from different parameters, p, in the TX boxes. To compensatefor these signal distortions compensating parameters, p′, are introducedin the compensating means 11, 13 where each compensating parameter, p′,is adapted to provide a signal distortion that substantially counteractsthe signal distortion provided by the corresponding parameter, p, in theTX boxes. There are a number of different ways to use these transmittingbranch models to adjust the compensation in the compensating means 11,13 such that the total signal distortion in the transmitting branches isminimised. One way to counteract the IQ-modulator errors is to digitallypredistort the IQ-signal at base band using iteratively updated filterstructures, for example an LMS algorithm, as described in the article“Digital Precompensation of Imperfections in Quadrature Modulators”, R.Marchesani, IEEE. Trans. on Communications, vol. 48, no. 4, April 2000,pp. 552-556. A standard implementation of IQ compensation isschematically shown in FIG. 4 which will be further described below.

The functions of the compensation adjustment means 17 are now describedin more detail with reference to FIG. 1 b. The compensation adjustmentmeans 17 comprises a first receiving means 31 adapted to receive theoutput signal z from the composite amplifier and a second receivingmeans 33 adapted to receive the input signal x. The first receivingmeans 31 and the second receiving means 33 are both connected to anerror deriving means 35 adapted to compare the output signal z with theinput signal x and derive an error representing the signal distortionsprovided in the transmitting branches 3, 5 entering the compositeamplifier 1. The first and second receiving means 31 and 33 can comprisemeans for adjusting the signals in relation to each other, for exampledelay-, phase-, and gain-adjustments. Alternatively this adjusting ofthe signals can be performed in the error deriving means 35. Thecompensation adjustment means 17 comprises further a signal derivationmeans 37 connected to the second receiving means 33. Said signalderivation means 37 is adapted to derive the input signals a1, a2 to therespective transmitting branches 3,5. The signal derivation means 37uses the input signal x and information about how the signal generatingmeans 6 works to derive the input signals a1 and a2. The compensationadjustment means 17 comprises furthermore a computation means 39connected to the signal derivation means 37 and to the error derivingmeans 35. The computation means 39 comprises a composite amplifier modelf and a transmitting branch model g1, g2 for each transmitting branchand uses these models together with the derived error and the derivedinput signals a1 and a2 to compute how much compensation parameters, p′,in the compensation means 11, 13 should be changed in order to minimisethe error in the output signal. This is an iterative process and thecomputation means 39 can in one embodiment derive the functions f, g1and g2 in respect of the compensating parameters to see in whichdirections the compensating parameters, p′, should be changed tominimise the error of the output signal. This will be described in moredetail below. The computation means 39 is further connected to aforwarding means 41 which is adapted to forward new compensatingparameters, p′, to the compensation means 11, 13 in accordance with thecomputations made by the computation means 39.

The method is further described with reference to the flow chart of FIG.2. In step S1 an input signal x is provided to the composite amplifier1. In step S2 the TOR 15 observes the output signal z from the compositeamplifier 1. This signal is forwarded to the compensation adjustmentmeans 17 in S3. In S5 the observed signal is compared with an idealoutput signal x in the compensation adjustment means 17 and an error ofthe observed signal z in relation to the ideal signal x is derived.Ideal signal means a theoretical signal travelling through thetransmitting branches not being effected by any signal distortions. Inthis example the ideal signal is also equal to the input signal x to thegenerating means 6. However more signal processing such as predistortioncan also be performed before the signal reaches the generating means 6.

In S9 the signal deriving means 37 utilises the known behaviour of thegenerating means 6 to derive the input signals a1 and a2 to thecompensation means 11, 13. These signals could also, optionally, be feddirectly from the transmission branches 3, 5 to the compensationadjustment means 17.

In S11 the computation means 39 derives correction steps for thecompensating parameters, p′, so that the error in the output signal isreduced. This computation involves utilising a composite amplifier modelto derive the contribution from each transmitting branch to the derivedsignal error. This composite amplifier model is a model of the behaviourof the composite amplifier, for example the contribution from thedifferent transmitting branches to the output signal for different inputsignal levels. If the composite amplifier for example is a Dohertyamplifier the composite amplifier model tells that only one of the twoamplifiers is driven during low signal levels as will be discussedfurther below. The computation further involves utilising a transmittingbranch model for each transmitting branch of the system in order toderive the contribution of the different parameters, p, of thetransmitting branch to the signal distortion giving rise to the observederror. This model comprises information about the contribution of thedifferent parameters in the transmitting branch to the signaldistortion. The main contribution to the signal distortion is theIQ-modulator but it could also be time delay or nonlinearities orfrequency errors. Further details on this computation of correctionsteps will be described below.

Depending on the composite amplifier model it could be necessary toprovide at least two different input signals such that the compositeamplifier is driven differently, i.e. the constituent amplifiers andthus the transmission branches are contributing to the output signal indifferent amounts. The input signals could then for example differ insignal levels or frequencies.

In S13 instructions are forwarded to the compensating means 11, 13 fromthe compensation adjustment means 17 regarding how compensatingparameters, p′, in the compensating means 11, 13 should be adjusted togive a signal distortion to compensate for the signal distortions causedby the parameters, p, in the TX boxes 7, 9.

This compensation adjustment described in relation to FIG. 2 could beperformed continuously or repeatedly or at certain intervals. Theadjustment of the compensating parameters is preferably an iterativeprocess such that the compensating parameters are adjusted to more andmore accurate values giving a smaller and smaller error. To achieve thisiteration the input signals provided to the system have to differ in forexample level or frequency as described above. Furthermore it could beadvantageous to be able to change the compensation parameters, p′, laterduring use of the system in response to for example temperature changeseffecting the parameters in the TX boxes 7, 9 or changes due to aging orother variations in the components or surrounding environment. Thecomposite amplifier model may also need to be updated over time if, forexample measurements show that the model is not correct.

One possible way to provide the composite amplifier model and thetransmitting branch models in the compensation adjustment means 17 is toexpress the output signal z as a function f of the signals y1 and y2input to the composite amplifier 1:z=f(y1, y2)

The function f is hereby the composite amplifier model. Furthermore y1and y2 are functions g1 and g2 of the input signals b1 and b2 to the TXboxes and of the parameters, p, in the TX boxes, here called p1 and p2in the first TX box 7 and p3 and p4 in the second TX box 9. Of coursethe number of parameters could vary.y1=g1(b1, p1, p2)y2=g2(b2, p3, p4)

The functions g1 and g2 are hereby the transmission branch models.Furthermore b1 and b2 are functions h1 and h2 of the input signals a1and a2 to the compensating means 11, 13 and of the compensatingparameters, p′, in the compensating means provided for giving an“inverted” signal distortion compared to the signal distortion providedby the parameters in the TX boxes. The compensating parameters are herecalled p1′ and p2′ for the first compensating means 11 and p3′ and p4′for the second compensating means 13.b1=h1(a1, p1′, p2′)b2=h2(a2, p3′, p4′)

The error measured in the output signal z should be minimised byproviding and stepwise changing the compensating parameters p1′, p2′,p3′ and p4′ such that the functions h1 and h2 are closer and closer tothe inverses of the functions g1 and g2. Therefore the compensationadjustment means 17 comprises computation means adapted to derive firstb1 in respect of p1′ and p2′ and b2 in respect of p3′ and p4′, then y1in respect of b1 and y2 in respect of b2 and then z in respect of y1 andy2. The input signals a1 and a2 are either known, or can be derived fromthe input signal x and the known behaviour of the generating means 6.Using this and performing said derivatives and comparing with theobserved output signal error will provide new compensating parameters p′to the compensating means 11, 13. From the above given equations andderivatives it can be found how the output signal z is effected by achange in the compensating parameters p1′, p2′, p3′ and p4′. Theequations will show in which direction the compensating parameters, p′,should be changed to minimise the error in the output signal z comparedto the ideal output signal x. This is done iteratively, i.e. thecompensating parameters, p′, are changed in small steps in the directiongiven from the derivatives and a minimum error of the output signal zwill be achieved. As said above it is necessary to provide differentinput signals to perform the iteration.

This is described in more detail below.

In order to compensate for the signal distortions in the TX boxes, weneed to know how the compensating parameters p′ affect the amplifieroutput. This information is contained in the partial derivatives of theoutput signal z with respect to the parameters p′. These derivatives canbe calculated using the “chain rule” as shown for p1′ below:

$\frac{\partial z}{\partial\left( {p\; 1^{\prime}} \right)} = {\frac{\partial z}{\partial\left( {y\; 1} \right)} \cdot \frac{d\left( {y\; 1} \right)}{d\left( {b\; 1} \right)} \cdot \frac{d\left( {b\; 1} \right)}{d\left( {p\; 1^{\prime}} \right)}}$

These derivatives can be used to calculate the required step in p′ usingseveral methods. Two of these, one using Newtons method and anotherusing an LMS-algorithm, are described below. Other options could be aKalman filter or an RLS-algorithm.

Using Newton's method the step Δp′ comes from solving the followingsystem of equations:F·Δp′=e

F is a matrix containing the partial derivatives for several differentinput signals and e is a vector containing corresponding values of theerror signal. Δp′ is the calculated step change of the parameters p′. Anexample with 5 different input signal samples is given below.

$F = \begin{bmatrix}\frac{\partial z_{1}}{\partial\left( {p\; 1^{\prime}} \right)} & \frac{\partial z_{1}}{\partial\left( {p\; 2^{\prime}} \right)} & \frac{\partial z_{1}}{\partial\left( {p\; 3^{\prime}} \right)} & \frac{\partial z_{1}}{\partial\left( {p\; 4^{\prime}} \right)} \\\frac{\partial z_{2}}{\partial\left( {p\; 1^{\prime}} \right)} & \frac{\partial z_{2}}{\partial\left( {p\; 2^{\prime}} \right)} & \frac{\partial z_{2}}{\partial\left( {p\; 3^{\prime}} \right)} & \frac{\partial z_{2}}{\partial\left( {p\; 4^{\prime}} \right)} \\\frac{\partial z_{3}}{\partial\left( {p\; 1^{\prime}} \right)} & \frac{\partial z_{3}}{\partial\left( {p\; 2^{\prime}} \right)} & \frac{\partial z_{3}}{\partial\left( {p\; 3^{\prime}} \right)} & \frac{\partial z_{3}}{\partial\left( {p\; 4^{\prime}} \right)} \\\frac{\partial z_{4}}{\partial\left( {p\; 1^{\prime}} \right)} & \frac{\partial z_{4}}{\partial\left( {p\; 2^{\prime}} \right)} & \frac{\partial z_{4}}{\partial\left( {p\; 3^{\prime}} \right)} & \frac{\partial z_{4}}{\partial\left( {p\; 4^{\prime}} \right)} \\\frac{\partial z_{5}}{\partial\left( {p\; 1^{\prime}} \right)} & \frac{\partial z_{5}}{\partial\left( {p\; 2^{\prime}} \right)} & \frac{\partial z_{5}}{\partial\left( {p\; 3^{\prime}} \right)} & \frac{\partial z_{5}}{\partial\left( {p\; 4^{\prime}} \right)}\end{bmatrix}$ ${\Delta\; p^{\prime}} = \begin{bmatrix}{\Delta\left( {p\; 1^{\prime}} \right)} \\{\Delta\left( {p\; 2^{\prime}} \right)} \\{\Delta\left( {p\; 3^{\prime}} \right)} \\{\Delta\left( {p\; 4^{\prime}} \right)}\end{bmatrix}$ $\underset{\_}{e} = \begin{bmatrix}{x_{1} - z_{1}} \\{x_{2} - z_{2}} \\{x_{3} - z_{3}} \\{x_{4} - z_{4}} \\{x_{5} - z_{5}}\end{bmatrix}$

The equation system above is usually overdetermined (as above) and canthen be approximately solved using a least-squares approximation. Thesystem can also be underdetermined, in which case there exist severalpossible solutions.

Another option for calculating the step in p′ is to use an LMS method inwhich steps are taken in the opposite direction of the partialderivatives:

${\Delta\left( {p\; 1^{\prime}} \right)} = {{{{- \mu} \cdot {Re}}\left\{ \frac{\partial|e|^{2}}{\partial\left( {p\; 1^{\prime}} \right)} \right\}} = {{2 \cdot \mu \cdot {Re}}\left\{ {e \cdot \left( \frac{\partial z}{\partial\left( {p\; 1^{\prime}} \right)} \right)^{*}} \right\}}}$where μ is a selectable parameter that controls the step-size andconvergence behaviour of the algorithm. A too large value of μ resultsin an unstable system that won't converge, while a small value of μresults in slow convergence.

Many variations of this algorithm exists, these can includenormalization and averaging over several input signals. Several stepswith different input signal will be necessary for the algorithm toconverge.

Another embodiment of the invention where a digital Doherty amplifier isthe composite amplifier will now be described. In this example onlysignal distortions from the IQ-modulator are considered. Otherparameters could however also in this example have been regarded. Thesystem comprising the digital Doherty amplifier is schematically shownin FIG. 3. Components being the same as in FIG. 1 are given also thesame reference numbers.

In this system a digital pre-distorter 21 is shown. A digitalpre-distorter is provided in order to compensate for signal distortionsin the amplifiers. The Composite amplifier 1 is in this example aDoherty amplifier and comprises thus a main amplifier 23 and anauxiliary amplifier 25. The main amplifier 23 is connected to a commonoutput of the composite amplifier via a quarter-wave transmission line27.

Furthermore a signal component separator 29 adapted to split and shapethe drive signals to the different transmission branches 3, 5 in orderto achieve high efficiency from the Doherty amplifier is connected tothe digital pre-distorter 21. The signal component separator 29 is inthis Doherty embodiment of the invention adapted to provide a phaseshift of substantially 90 degrees to the second transmission branch 5 inorder to compensate for the phase shift provided by the quarter-wavetransmission line 27 to the first transmission branch 3.

The composite amplifier model used by the compensation adjustment means17 in this embodiment of the invention says thus that according to thecharacteristics of a Doherty amplifier only the main amplifier 23 isdriven at low input signal levels. This information is used by thecompensation adjustment means 17 such that the observed error in theoutput signal is totally assigned to the first transmitting branch 3when low input signals are provided. Hereby the first compensating means11 can first be adjusted according to this and according to atransmitting branch model as described above. Then according to thecomposite amplifier model a higher input signal level is provided andboth the amplifiers are driven. The error observed now originates onlyfrom signal distortions in the second transmitting branch since thesignal distortion coming from the main amplifier branch already has beencompensated for. This is used together with the second transmittingbranch model to adjust the parameters in the second compensating means13.

Instead of first compensating the first transmitting branch and thenprovide a higher input level and compensate the second transmittingbranch it is also possible to first derive the errors related from boththe transmitting branches and then compensate both the branchessimultaneously. This is possible since the error relating from the firsttransmitting branch is known from the measurements with low input signallevels and this error can be subtracted from the error observed when ahigher input signal level is provided and both amplifiers are driven.The error from the first transmission branch is then assumed to riselinearly with signal level.

This embodiment is described in more detail below.

In FIG. 4 a standard implementation of IQ compensation is schematicallyshown. G_(II), G_(QQ), G_(QI) and G_(IQ) correspond to the compensatingparameters p′ in the function h described above. They compensate for theIQ error, whereby G_(II) and G_(QQ) compensate for gain imbalance andG_(QI) and G_(IQ) compensate for phase imbalance. Furthermore the termsDC_(I) and DC_(Q) compensate for DC-offset and CO-leakage. Thecorresponding equation for the first compensation means 11 (13 issimilar) is:b1=G _(II) ·Re {a1}+G _(IQ) ·Im {a1}+DC _(I) +j·G _(IQ) ·Re {a1}+j·G_(QQ) ·Im {a1}+j·DC _(Q)

The error from the first transmitting branch, here called the mainbranch (nonlinearity of the main amplifier and the IQ modulator error ofthe main branch and other parameters) can be estimated for the entiresignal interval below a threshold where the auxiliary amplifier isturned on.

The error from the second transmitting branch, here called the auxiliarybranch (nonlinearity of the auxiliary amplifier, the IQ modulator errorof the auxiliary branch and other parameters) has to be estimated on thesignal levels above the threshold where the auxiliary amplifier isturned on. The composite error seen at the output of the Dohertyamplifier at those power levels will have contributions from both themain branch and the auxiliary branch.

The auxiliary IQ error will be seen predominantly with theapproximately, 90 degrees of phase shift compared to the error from themain branch.

This solution uses direct inverse IQ compensation adaptation using onlythe signal Reference signal x (before the DPD block 21) and theObservation signal z (observed in the TOR 15). The adaptation is madeiterative using a LMS-like algorithm.

By using the phase shift of 90 degrees and power selection on theauxiliary part, the adaptation can be made on all parameterssimultaneously. The z signal is assumed to be average time and phasealigned and sometimes also gain aligned with the x signal before the IQand DC parameter estimation.

The equations for the adaptation of the main IQ error are:

$G_{{QQ},{i + 1}} = {G_{{QQ},i} + {\mu_{M} \cdot \frac{{\underset{k}{\Sigma}\left( {{{Im}{\left\{ {x - z} \right\} \cdot {Im}}\left\{ x \right\}} - {{Re}{\left\{ {x - z} \right\} \cdot {Re}}\left\{ x \right\}}} \right)} \cdot \delta}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \delta} \right.}}}$$G_{{II},{i + 1}} = {G_{{II},i} - {\mu_{M} \cdot \frac{{\underset{k}{\Sigma}\left( {{{Im}{\left\{ {x - z} \right\} \cdot {Im}}\left\{ x \right\}} - {{Re}{\left\{ {x - z} \right\} \cdot {Re}}\left\{ x \right\}}} \right)} \cdot \delta}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \delta} \right.}}}$$G_{{QI},{i + 1}} = {{G_{{QI},i} + {{\mu_{M} \cdot \frac{{\underset{k}{\Sigma}\left( {{{Im}{\left\{ {x - z} \right\} \cdot {Re}}\left\{ x \right\}} + {{Re}{\left\{ {x - z} \right\} \cdot {Im}}\left\{ x \right\}}} \right)} \cdot \delta}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \delta} \right.}}G_{{IQ},{i + 1}}}} = {G_{{IQ},i} + {\mu_{M} \cdot \frac{{\underset{k}{\Sigma}\left( {{{Im}{\left\{ {x - z} \right\} \cdot {Re}}\left\{ x \right\}} - {{Re}{\left\{ {x - z} \right\} \cdot {Im}}\left\{ x \right\}}} \right)} \cdot \delta}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \delta} \right.}}}}$${dc}_{I,{i + 1}} = {{dc}_{I,i} + {\mu_{dcM} \cdot \frac{\underset{k}{\Sigma}{Re}{\left\{ {x - z} \right\} \cdot \delta}}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \delta} \right.}}}$${dc}_{Q,{i + 1}} = {{dc}_{Q,i} + {\mu_{dcM} \cdot \frac{\underset{k}{\Sigma}{Im}{\left\{ {x - z} \right\} \cdot \delta}}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \delta} \right.}}}$where δ=1 for |x|≦“the transition point for this specific Dohertyamplifier” and δ=0 for |x|>“the transition point for this specificDoherty amplifier”, μ_(M)is the step length for the adaptation of themain IQ compensation parameters and the μ_(dcM) is the step length inthe adaptation of main dc offset. These equations are a variation of theLMS equations described above, with the addition of normalization (thedenominator), to make the step-size invariant of the input signalenergy, and averaging (summing over several samples) to make the stepless noisy.

The numerator (except the averaging) can be derived from the earlierequations by making some approximations. First, the composite amplifiermodel is assumed to be one for small signals, and zero for largesignals, as given by the equation below (with γ=1−δ).

f(y 1, y 2) ≈ δ ⋅ y 1 + γ ⋅ 𝕖^((j π/2))y 2$\frac{\partial z}{\partial\left( {y\; 1} \right)} \approx \delta$

Then the TX-box, 7, is assumed to only give small signal distortionswhich allows us to ignore them in the derivative:

$\frac{d\left( {y\; 1} \right)}{d\left( {b\; 1} \right)} \approx 1$

The pre-distorter, 21, and signal component separator, 29, is alsoassumed to only give a small signal distortion, so we can approximate alwith x. Further, we assume that we don't want to correct for gain andphase errors in the IQ-compensator. This is achieved by settingG_(II)=1+q, G_(QQ)=1−q and G_(QI)=G_(IQ)=r. The rest of the derivativecan be extracted from the equation corresponding to FIG. 4, which givesus a complete expression for the partial derivatives of the outputsignal with respect to the parameters as follows:

$\frac{\partial z}{\partial(q)} \approx {x^{*} \cdot \delta}$$\frac{\partial z}{\partial(r)} \approx {j \cdot x^{*} \cdot \delta}$$\frac{\partial z}{\partial\left( {DC}_{I} \right)} \approx \delta$$\frac{\partial z}{\partial\left( {DC}_{I} \right)} \approx {j \cdot \delta}$

This, together with the LMS equations above, and the normalization andaveraging gives the update equations above.

The equations for the adaptation of the auxiliary IQ error are:

$G_{{QQ},{i + 1}} = {G_{{QQ},i} + {\mu_{A} \cdot \frac{{\underset{k}{\Sigma}\left( {{{Im}{\left\{ {x - z} \right\} \cdot {Im}}\left\{ {x\;{\mathbb{e}}^{({{- j}\;\pi})}} \right\}} - {{Re}{\left\{ {x - z} \right\} \cdot {Re}}\left\{ {x\;{\mathbb{e}}^{({{- j}\;\pi})}} \right\}}} \right)} \cdot \gamma}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \gamma} \right.}}}$$G_{{II},{i + 1}} = {G_{{II},i} - {\mu_{A} \cdot \frac{{\underset{k}{\Sigma}\left( {{{Im}{\left\{ {x - z} \right\} \cdot {Im}}\left\{ {x\;{\mathbb{e}}^{({{- j}\;\pi})}} \right\}} - {{Re}{\left\{ {x - z} \right\} \cdot {Re}}\left\{ {x\;{\mathbb{e}}^{({{- j}\;\pi})}} \right\}}} \right)} \cdot \gamma}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \gamma} \right.}}}$$G_{{QI},{i + 1}} = {{G_{{QI},i} + {{\mu_{A} \cdot \frac{{\underset{k}{\Sigma}\left( {{{Im}{\left\{ {x - z} \right\} \cdot {Re}}\left\{ {x\;{\mathbb{e}}^{({{- j}\;\pi})}} \right\}} + {{Re}{\left\{ {x - z} \right\} \cdot {Im}}\left\{ {x\;{\mathbb{e}}^{({{- j}\;\pi})}} \right\}}} \right)} \cdot \gamma}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \gamma} \right.}}G_{{IQ},{i + 1}}}} = {G_{{IQ},i} + {\mu_{A} \cdot \frac{{\underset{k}{\Sigma}\left( {{{Im}{\left\{ {x - z} \right\} \cdot {Re}}\left\{ {x\;{\mathbb{e}}^{({{- j}\;\pi})}} \right\}} + {{Re}{\left\{ {x - z} \right\} \cdot {Im}}\left\{ {x\;{\mathbb{e}}^{({{- j}\;\pi})}} \right\}}} \right)} \cdot \gamma}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \gamma} \right.}}}}$${dc}_{I,{i + 1}} = {{dc}_{I,i} + {\mu_{dcA} \cdot \frac{\underset{k}{\Sigma}{Re}{\left\{ {\left( {x - z} \right){\mathbb{e}}^{({{- j}\;\pi\text{/}2})}} \right\} \cdot \gamma}}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \gamma} \right.}}}$${dc}_{Q,{i + 1}} = {{dc}_{Q,i} + {\mu_{dcA} \cdot \frac{\underset{k}{\Sigma}{Im}{\left\{ {\left( {x - z} \right){\mathbb{e}}^{({{- j}\;\pi\text{/}2})}} \right\} \cdot \gamma}}{\left. \underset{k}{\Sigma} \middle| x \middle| {}_{2}{\cdot \gamma} \right.}}}$Where:

-   γ=1 for |x|>“the transition point for this specific Doherty    amplifier” and γ=0 for |x|≦“the transition point for this specific    Doherty amplifier”, μ_(A) is the step length for the adaptation of    the auxiliary IQ compensation parameters and the μ_(dcA) is the step    length in the adaptation of auxiliary dc offset. We can derive these    equations in a manner similar to the main equations by making the    following approximations and observations:

$\frac{\partial z}{\partial\left( {y\; 2} \right)} \approx {\gamma \cdot {\mathbb{e}}^{j\;\pi\text{/}2}}$$\frac{d\left( {y\; 2} \right)}{d\left( {b\; 2} \right)} \approx 1$a 2 ≈ x ⋅ 𝕖^(−j π/2)

The phase shifts above comes from the phase shift in the amplifier (weassume the phase alignment is made for the main branch) and from thephase shift in block 6. These phases are here assumed to be +/−90degrees respectively, but they can be something else. A phase offset inthe second TX box 9 relative to the first TX box 7 would for examplechange the shift in the first equation above. We can now use theseapproximations and formulate the complete derivatives for the auxiliarybranch:

$\frac{\partial z}{\partial(q)} \approx {x^{*} \cdot {\mathbb{e}}^{j\;\pi} \cdot \gamma}$$\frac{\partial z}{\partial(r)} \approx {j \cdot x^{*} \cdot {\mathbb{e}}^{j\;\pi} \cdot \gamma}$$\frac{\partial z}{\partial\left( {DC}_{I} \right)} \approx {\gamma \cdot {\mathbb{e}}^{j\;\pi\text{/}2}}$$\frac{\partial z}{\partial\left( {DC}_{I} \right)} \approx {j \cdot \gamma \cdot {\mathbb{e}}^{j\;\pi\text{/}2}}$

Again, we combine these with the LMS equation above and add averagingand normalization to get to the update equations above.

These equations provide a parameter compensation in both I and Q.Especially when a DPD is used this compensation is redundant. If minimumredundancy is desired only four of these parameters have to beindividually adjusted, i.e. DC_(I) and DC_(Q), one of G_(II) or G_(QQ)and one of G_(QI) or G_(IQ).

The auxiliary amplifier is in practice visible in the output, althoughsubstantially less than the main amplifier, at all levels due to circuitparasitics. The amount of correction that can be achieved (in the firstpass) thus depends on the size of the circuit parasitics relative to thenonlinear part of the main branch IQ-errors.

The auxiliary amplifier can be made more visible in the output signal inthe upper region, by adjusting the amount of the main amplifier'ssaturation. This saturation, in the extreme, makes the main amplifierlook like a short circuit. The quarter-wave line then transforms thisinto an open circuit at the auxiliary amplifier's output. The auxiliaryamplifier's output current is then fully visible as a voltage at theoutput (load). Saturating the main amplifier thus magnifies theIQ-errors due to the auxiliary amplifier. This can be used to betterobserve them in the upper region. The change to the simple modeldescribed above is only in the visibility of the auxiliary amplifieroutput.

In FIG. 5 a flow chart of the method according to the embodimentdescribed in relation to FIG. 3 is shown. In step S31 a low input signallevel below a threshold for the auxiliary amplifier is provided to thesystem. Hereby only the main amplifier is driven. In step S33 thecompensation adjustment means 17 derives an error originating from thesignal distortions in the main branch by comparing the observed signal zforwarded from the TOR 15 with an ideal signal x as described above. Instep S35 an input signal level above the threshold for the auxiliaryamplifier is provided. Hereby both the main amplifier and the auxiliaryamplifier are driven. In S37 the error originating form the auxiliarybranch is derived in the compensation adjustment means 17 by comparingthe observed output signal forwarded from the TOR 15 with an idealsignal as above and subtracting the error coming from the main branch asderived in the step S33 and assuming this error is linearly rising withpower.

In step S39 the compensation adjustment means 17 calculates according tothe equations given above new compensating parameters and in S41 thesenew compensating parameters are forwarded to the first and secondcompensating means 11, 13. As described above this is done iteratively.Therefore the whole process is started from the beginning in step S31and new compensating parameters are calculated for minimising the error.

If the composite amplifier comprises more than two transmitting branchinputs and possibly also more than two constitute amplifiers thecomposite amplifier model is of course more complicated than for aDoherty amplifier. The model could then for example include informationabout the contribution of the different transmitting branches to theoutput signal for many different input signal levels or frequencies.Hereby a combination of all this information will provide a possible wayto single out the contribution from each branch.

The composite amplifier in FIG. 1 can according to the invention be aChireix amplifier. A Chireix amplifier comprises a first amplifierconnected to the first transmission branch 3 and a second amplifierconnected to the second transmission branch 5. The amplifiers areconnected to the output of the composite amplifier through twoquarter-wave lines λ/4 and two compensating reactances +jX and −jX,which are used to extend the region of high efficiency to include loweroutput power levels. The efficiency of Chireix systems is analyzed infor example F. H. Raab, “Efficiency of outphasing RF Power AmplifierSystems”. IEEE Trans. Communications, vol. COM-33, no. 10, pp.1094-1099, October 1985.

Chireix amplifiers traditionally use equal-amplitude drive signals, andthe IQ-modulator errors from both branches are thus equally visible inthe same output level ranges. Generally this calls for a more advancedmodel of the amplifier, i.e. a more detailed expression of the functionf above. But, either constituent amplifier can be driven by itself (withretained efficiency) at low output levels (below some transition point).The previously described method for Doherty amplifiers can then be usedwithout alteration, except for the phase shifts, for Chireix amplifiers.The model used can then also be as simple as that of the previousDoherty example.

There is, in Chireix amplifiers, also the further possibility to firstdrive one amplifier at low levels and observe the IQ-errors from it,then change so that the other amplifier is the one driven at low levelsand observe its IQ-errors. The Chireix amplifier, due to its balancedproperties, thus has an extra possibility for individual observation ofIQ-errors from the different branches. Note that this observationpossibility can be achieved in the other composite amplifiers as well,but at a higher cost, since the other constituent amplifiers are not asefficient at low levels as the one that is dedicated for operation inthis region.

Composite amplifiers in general can be handled by either providing anexpression for derivates of the function f with respect to the inputsignals, or by resolving the IQ-errors from a first constituentamplifier by observing them at low levels, proceeding with a nextamplifier in a higher output level region, and so on. By using theChireix drive separation trick above, all composite amplifiers'IQ-errors can in principle be handled in succession, starting from lowoutput levels.

FIG. 6 shows an embodiment of the invention where the compositeamplifier 1′ is a Chireix-Doherty amplifier in accordance with WO2004/057755. This embodiment comprises a predistorter, 21, a signalcomponent separator, 29, that separates the input signal x into a first,a second and a third input signal a1, a2, a3 to a first, a second and athird transmission branch 43,45 and 46 respectively. The transmissionbranches comprises a first, a second and a third compensation means 111,113 and 114 respectively that according to the invention substantiallycompensates for signal distortion in a first, a second and a third TXbox 117,119 and 120 respectively. The composite amplifier 1′ comprises afirst amplifier 103 a, a second amplifier 103 b and a third amplifier103 c, where the first amplifier 103 a is connected to the first TX box117, the second amplifier 103 b is connected to the second TX box 119and the third amplifier 103 c is connected to the third TX box 120. Thecomposite amplifier 1′ comprises further a first transmission line 105provided between the first amplifier 103 a and the output and a secondtransmission line 106 provided between the third amplifier 103 c and theoutput. One of these transmission lines is slightly shorter than aquarter wavelength and the other is slightly longer so that the firstand third amplifiers 103 a and 103 c form a Chireix pair. The secondamplifier 103 b is a peak amplifier. The compensation adjustment means17 uses the observed output signal z, from the transmitter observationreceiver 15, together with the ideal output signal x to calculate theadjustment steps to the compensating parameters p′, in the compensationmeans 111, 113 and 114 so that the observed error in the output signalis minimized.

The calculations of the adjustments to the compensating parameters p′ isdone utilizing a model of the amplifierz=f(y1,y2,y3)

This model is derived with respect to its input signals y1, y2 and y3.The derivate of these input signals with respect to the parameters p′are also calculated and used to adjust the compensating parameters inthe way described above. It is possible to utilize signal regions whereonly some of the amplifiers are visible in the output to simplify thederivates. For example, the peak amplifier 103 b is only visible at highoutput signals and the Chireix pair 103 a and 103 c can be driven insuch a way that only one is visible at low output levels. At mediumoutput levels both of the Chireix amplifiers are visible at the output.

The invention claimed is:
 1. A method for compensating signaldistortions in multiple transmitting branches entering a compositeamplifier, the method comprising: providing an input signal to thecomposite amplifier; observing an output signal from the compositeamplifier; deriving an error in the output signal by comparing theoutput signal with an ideal output signal; deriving the individualcontribution from each transmitting branch to the error by utilising acomposite amplifier model, said composite amplifier model comprisinginformation about the contribution from each constituent amplifier tothe output signal for each provided input signal; compensating thesignal distortions in the transmitting branches accordingly; derivingthe contribution from different parameters in the transmitting branchesto the signal distortion that causes the observed error by utilising atransmitting branch model comprising information about the parametersthat affect signal distortions in each transmitting branch; andproviding compensating parameters or adjustments to already existingcompensating parameters to each transmitting branch accordingly in orderto decrease the signal distortions.
 2. The method according to claim 1,further comprising: deriving derivatives of the output signal in respectof the different compensating parameters by utilising said compositeamplifier model and said transmitting branch models; utilising saidderivatives for adapting said compensating parameters such that theerror in the output signal is minimised.
 3. The method according toclaim 1, further comprising providing at least two different inputsignals levels or frequencies such that the different transmissionbranches contributes in different amounts to the output signal for thedifferent input signals.
 4. The method according to claim 1, furthercomprising: providing a first input signal level to the compositeamplifier such that generally only one of the constituent amplifiers isdriven, said amplifier hereafter being called the first amplifier;relating the derived error to a transmitting branch connected to thefirst amplifier; providing a second input signal level such that asecond amplifier is driven either alone or together with the firstamplifier; deriving a signal error caused by a transmitting branchconnected to the second amplifier by observing the output signal andsubtracting a surmised linearly rising error caused by the firsttransmitting branch; and compensating the observed signal distortions byproviding compensating means in each branch of the composite amplifier.5. The method according to claim 1, further comprising iterating themethod steps in order to obtain gradually improved compensations of thesignal distortions.
 6. The method of claim 1, wherein the method furthercomprises: using the ideal output signal to produce a first signal and asecond signal; producing a first compensation parameter and a secondcompensation parameter; providing the first signal to a firstcompensating circuit configured to distort the first signal based on thefirst compensation parameter, thereby producing a first distortedsignal; providing the second signal to a second compensating circuitconfigured to distort the second signal based on the second compensationparameter, thereby producing a second distorted signal; providing thefirst distorted signal to a first transmission circuit comprising afirst modulator, wherein the first transmission circuit is configured touse the first distorted signal and the first modulator to generate thefirst input signal, wherein the first input signal is a modulatedsignal; providing the second distorted signal to a second transmissioncircuit comprising a second modulator, wherein the second transmissioncircuit is configured to use the second distorted signal and the secondmodulator to generate the second input signal, wherein the second inputsignal is a modulated signal, and the step of providing an input signalto the composite amplifier comprises providing the first input signaland the second input signal to the composite amplifier.
 7. The method ofclaim 6, the step of producing the first and second compensationparameters comprises: (i) comparing the output signal with the idealoutput signal to produce an error signal and (ii) determining the firstand second compensation parameters using the error signal and thecomposite amplifier model.
 8. A system comprising a composite amplifierand at least two transmitting branches entering the composite amplifier,said transmitting branches comprising parameters that cause signaldistortions, said system further comprising a transmitter observationreceiver (TOR) connected to the output of the composite amplifier,wherein the system further comprises: a compensation adjustment circuitconnected to the TOR and adapted to derive a signal error of the outputsignal from the composite amplifier by comparing the output signal withan ideal output signal, said compensation adjustment circuit furtherbeing adapted to derive the individual contribution to the error fromeach transmitting branch by using a composite amplifier model comprisinginformation about the contribution from each constituent amplifier tothe output signal for each provided input signal; and a compensatingcircuit in each transmitting branch, said compensating circuit beingconnected to the compensation adjustment circuit and adapted to receivetherefrom instructions regarding how the signal distortions in eachtransmitting branch should be compensated, wherein said at least twotransmitting branches comprises a first transmitting branch and a secondtransmitting branch, the compensation adjustment circuit furthercomprises a first transmitting branch model for the first transmittingbranch and a second transmitting branch model for the secondtransmitting branch, the models comprising information about thecontribution from different parameters of the transmitting branches tothe signal distortion that causes the observed error, and saidcompensation adjustment circuit is adapted to use the first and secondtransmitting branch models to derive said instructions and saidinstructions comprising compensating parameters or adjustments toalready existing compensating parameters to be forwarded to eachcompensating circuit in order to decrease the signal distortions.
 9. Thesystem according to claim 8, wherein the compensation adjustment circuitis adapted to derive the output signal in respect of the differentcompensating parameters by utilising said composite amplifier model andsaid transmitting branch models, said derivatives being indicative ofthe direction in which the compensating parameters should be changed inorder to minimise the measured error.
 10. The system according to claim8, wherein the compensation adjustment circuit is adapted to firstderive an error in the output signal relating only to a firsttransmitting branch connected to a first amplifier when an input signallevel to the system is provided such that generally only the firstamplifier is driven and then derive an error caused by a secondtransmitting branch connected to a second amplifier when a second inputsignal level is provided to the system such that the second amplifier isdriven either alone or together with the first amplifier by observingthe output signal and subtract a surmised linearly rising error causedby the first transmitting branch.
 11. The system according to claim 8,wherein the compensation adjustment circuit is adapted to iterate themethod steps in order to obtain gradually improved compensations of thesignal distortions.
 12. The system of claim 8, wherein the system isconfigured such that the compensation adjustment circuit receives twosignals and only two signals, said two signals being the ideal outputsignal and the output signal from the composite amplifier.
 13. Thesystem of claim 8, wherein the at least two transmitting branchescomprises a first transmitting branch and a second transmitting branch,the system further comprises a first signal generator configured togenerate from the ideal output signal a first input signal for inputtingto the first transmitting branch and a second input signal for inputtingto the second transmitting branch, and the compensation adjustmentcircuit comprises a second signal generator configured to generate afirst output signal and a second output signal, wherein the first outputsignal is substantially identical to the first input signal and thesecond output signal is substantially identical to the second inputsignal.
 14. The system of claim 13, wherein the second signal generatoris configured to receive the ideal output signal, and the second signalgenerator is further configured to generate the first and second outputsignal using the ideal output signal and information about the firstsignal generator.
 15. A compensation adjustment apparatus adapted to beconnected to an output of a composite amplifier and to at least twocompensation circuits provided in one each of at least two transmissionbranches entering the composite amplifier, the compensation adjustmentapparatus comprising: a first receiver adapted to receive an outputsignal from the composite amplifier; an error deriving circuit connectedto the first receiver and adapted to derive an error in the outputsignal caused by signal distortions in the transmitting branches bycomparing the output signal with an ideal output signal; computationcircuit connected to the error deriving circuit and adapted to derivethe individual contribution from each transmitting branch to the errorby using a composite amplifier model comprising information about thecontribution from each constituent amplifier to the output signal foreach provided input signal; and forwarding circuit connected to thecomputation circuit and adapted to forward instructions from thecomputation circuit to the compensation circuit regarding how the signaldistortions in each transmitting branch should be compensated, whereinthe computation circuit further comprises a transmitting branch modelfor each transmitting branch comprising information about thecontribution from different parameters of the transmitting branch to thesignal distortion that causes the observed error, and in that thecomputation circuit is adapted to use these transmitting branch modelsto derive said instructions and said instructions comprisingcompensating parameters or adjustments to already existing compensatingparameters to be forwarded to each compensating circuit in order todecrease the signal distortions.
 16. The apparatus of claim 15, whereinthe computation circuit is adapted to derive the output signal inrespect of the different compensating parameters by utilising saidcomposite amplifier model and said transmitting branch models, saidderivatives being indicative of the direction in which the compensatingparameters should be changed in order to minimise the measured error.17. The apparatus of claim 15, wherein the computation circuit isadapted to first derive an error in the output signal relating only to afirst transmitting branch connected to a first amplifier when an inputsignal level to the system is provided such that in principal only thefirst amplifier is driven and then derive an error caused by a secondtransmitting branch connected to a second amplifier when a second inputsignal level is provided to the system such that the second amplifier isdriven either alone or together with the first amplifier by observingthe output signal and subtract an assumed linearly rising error causedby the first transmitting branch.
 18. The apparatus of claim 15, whereinthe computation circuit is adapted to iterate the computations in orderto obtain gradually improved compensations of the signal distortions.19. The apparatus of claim 15, wherein the apparatus further comprises asecond receiver for receiving the ideal output signal, and the apparatusdoes not include any other receiver other than the first receiver andthe second receiver.
 20. The apparatus of claim 15, wherein the leasttwo transmission branches comprises a first transmission branchconfigured to receive a first input signal generated by a first signalgenerator and a second transmission branch configured to receive asecond input signal generated by the first signal generator, thecompensation adjustment apparatus further comprises a second signalgenerator configured to generate a first output signal and a secondoutput single, wherein the first output signal is substantiallyidentical to the first input signal and the second output signal issubstantially identical to the second input signal.
 21. The compensationadjustment apparatus of claim 20, wherein the second signal generator isconfigured to receive the ideal output signal, and the second signalgenerator is further configured to generate the first and second outputsignals using the ideal output signal and information about the firstsignal generator.
 22. A data transmission method, comprising: generatinga first signal (a1) and a second signal (a2) from an input signal (x);producing a first distorted signal by distorting the first signal (a1)based on a first compensation parameter; producing a second distortedsignal by distorting the second signal (a2) based on a secondcompensation parameter; generating a first modulated signal using the afirst modulator and the first distorted signal; generating a secondmodulated signal using a second modulator and the second distortedsignal; using the first and second modulated signals to produce anoutput signal (z); generating an error signal based on a comparison ofthe input signal (x) with the output signal (z); determining the firstand second compensation parameters using the error signal and acomposite amplifier model; and outputting the determined first andsecond compensation parameters.
 23. The data transmission apparatus ofclaim 22, wherein determining the first and second compensationparameters comprises using the error signal, the composite amplifiermodel, a first transmitting branch model corresponding to the firsttransmission circuit, and a second transmitting branch modelcorresponding to the second transmission circuit to determine the firstand second compensation parameters.